Preheat the oven's grill and set the oven rack about 15cm from the heat source.
Bring a large saucepan of lightly-salted water to a boil then cook the penne uncovered stirring occasionally until tender yet firm to the bite; about 11 minutes. Drain and return to the saucepan.
Meanwhile heat 1 tablespoon of the olive oil in a large frypan over medium-high heat. Cook and stir the sausage in the hot oil until crumbly and no longer pink; about 7 minutes. Remove from the frypan leaving the fat in the frypan and set aside.
Pour the remaining 2 tablespoons of olive oil into the frypan then add the mushrooms and garlic. Cook and stir until the mushrooms have browned and begun to release their liquid; about 5 minutes more.
Return the sausage to the frypan and pour in the cream. Season with salt and pepper then return to a simmer cooking until the sauce begins to thicken; about 5 minutes more.
Pour the sauce over the pasta and stir. Pour into a 23x33cm baking dish and sprinkle with mozzarella cheese.
Grill in the preheated oven until the mozzarella cheese is bubbly and golden brown; 5 to 7 minutes.